There may be more about it later in canon (or they may not be? I don't actually remember), but imo it's probably never been only because of Kunlun, but it's never only his free choice either. He is a genuinely good person who wants to protect everyone, but at the same time the promise is always in the picture. /Just me going from vibes, and not remembering what happens later re: this storyline, if anything. :D